1. Field of the Invention
This invention relates to an apparatus for forming a deposited film useful for uses of functional film, particularly semiconductor device, photosensitive device for electrophotography, electronic device such as optical input sensor device for optical image inputting device, etc.
2. Related Background Art
In the prior art, for example, as a photovoltaic device, a device having a photoelectric converting layer constituted of an amorphous material made of silicon atoms as the matrix, namely so called amorphous silicon (hereinafter represented as "a-Si") , on a substrate has been known.
As the method for forming a-Si film constituting the photoelectric converting layer of such photovoltaic device, some methods have been proposed, including the vacuum vapor deposition method, the ion plating method, the reactive sputtering method, the thermal CVD method, the plasma CVD method, the optical CVD method, etc. Among them, the plasma CVD has been practically applied as the most suitable method, and generally widely known.
Whereas, the photoelectric converting layer constituted of a-Si of the prior art, for example, one obtained by the plasma CVD method is enriched in exhibiting characteristics and has been accepted as satisfactory for the time being, but yet problems remain before solving the task of satisfying comprehensively all of the points required for establishing sure products, including the points of electrical, optical, photoconductive characteristics, fatigue resistance characteristics by repeated uses, use environment characteristic, the points of stability and durability with lapse of time, and further the point of homegeneity. This is greatly because the desired photoelectric converting layer is not a type which can be obtained by simple layer deposition operations, but above all skilled contrivances are required in the operational steps, although the materials employed may involve some problems
In this connection, for example, in the case of the thermal CVD method, after dilution of a silicon type gaseous material, the so called impurity is mixed and then the materials are pyrolyzed at a high temperature of 500.degree. to 650.degree. C. Accordingly, for formation of a desired a-Si film, precise step operation and control are demanded, whereby the device becomes complicated and considerably higher in cost. Even so, it is extremely difficult to obtain steadily photoelectric converting layers constituted of a-Si which is homogeneous and has the desired characteristics as mentioned above, and therefore such device can be employed on an industrial scale with difficulty.
Also, in the plasma CVD method generally widely used as the best method as described above, there exist some problems in step operations, and also problems in installation investment. As to the step operations, their conditions are further more complicated than the thermal CVD method as described above and can be generalized with extreme difficulty. For example, by referring merely to the relationship parameters of substrate temperature, flow rates and flow rate ratios of the gases introduced, pressure during layer formation, high frequency power, electrode structure, structure of reaction vessel, evacuation speed, plasma generating system, there already exist so many parameters. In addition, other parameters still exist, and therefore strict selection of parameters is required for obtaining a desired product and, because of strictly selected parameters, if one of the constituent factors, above all plasma may become unstable state, the film formed will be remarkably badly affected to give no acceptable product. And, as to the device, since strict selection of parameters is required as described above, its structure will become of itself complicated, and it must be designed so as to correspond to the parameters individually selected strictly according to the change in the scale and the kind of the device. For such reasons, the plasma CVD method, which has been presently accepted the best method, requires enormous installation cost for the device if bulk production of desired a-Si film is to be performed. Even by doing so, there are so many and complicated step management items for bulk production, with the step management tolerance being narrow and also the device control being severe. Consequently, the product obtained are considerably high in cost.
On the other hand, photovoltaic devices are becoming diversified, and it is socially demanded to supply photovoltaic devices having photoelectric converting layers constituted of stable a-Si films satisfying comprehensively the requirements of various characteristics, etc., as mentioned above, and also corresponding to the objects, uses to be applied, and which are sometimes made to have enlarged area, at low cost and steadily, and there is the situation in which development of the method and the device satisfying this demand is earnestly desired.
These are also applicable to other layers, for example, layers comprising a-Si film layer containing at least one selected from among oxygen atoms, carbon atoms, and nitrogen atoms.
Also, as another problem of the device for forming deposited film of the prior art, when effecting, for example, etching as the treatment before or after formation of deposited film, if treatment is conducted by use of separate devices corresponding to the respective treatments, the material to be treated is attached with dust to give rise to flaws, which caused badness such as leak after formed into a device.
On the other hand, when a device having desired performance is prepared by use of an amorphous semiconductor, it can be realized by laminating layers different in electroconductivity, activation energy or optical band gap such as p-type impurity conductive layer, n-layer impurity conductive layer, genuine semiconductor layer, narrow band gap layer, wide band gap layer, etc., by using, for example, the plasma CVD method and varying the gas composition.
For the purpose of effecting good film formation without lowering the performance of the device with the laminated constitution, it is desirable to perform film formation continuously without breaking vacuum during lamination in order to prevent the interfaces from formation of unnecessary levels. Also, it is necessary to prevent entrainment of the doping gas, etc., in the preceeding steps as the impurity. For this purpose, by making avail of the characteristics of the amorphous material with great tolerance of substrate selectivity, in forming the above lamination type semiconductor film on a flexible lengthy film such as plastic or metal plate, the so called Roll-to-Roll method has been employed. This method, as disclosed in U.S. Pat. No. 4,389,970, comprises arranging films such of stainless steel which can be wound up in rolls so as to be wound up through some film forming chambers, shielding the whole system from the air, and conveying the film at a predetermined speed through film forming chambers while winding up the film, thereby laminating successively necessary layers on the film which is the substrate.
FIG. 1 illustrates the basic constitution of the principal portions of a preferred embodiment of the bulk production type vacuum film forming device according to the plasma CVD method of the prior art which laminates p, i, n type semiconductor layers by the Roll-to-Roll method, which is described in detail below.
111 is a substrate and it is moved continuously from the delivery roller to the take-up roller 113.
The laminated film forming chambers 114, 115, 116 are evacuated by evacuating pipes 117, 118, 119 to be maintained at vacuum.
The substrate 111 is heated by the heaters 120, 121, 122 within the deposited film forming chambers 114, 115, 116 for depositing p, i, n type semiconductor layers.
The starting material gases for film formation such as SiH.sub.4, B.sub.2 H.sub.6, PH.sub.3, CH.sub.4, etc., and inert gases such as Ar, He, etc., are introduced through the gas introducing pipes 126, 127, 128 and the inner pressures are controlled suitably by the valves 123, 124, 125.
Further, high frequency voltages are applied by power sources 132, 133, 134 on the electrodes 129, 130, 131 to generate plasma and form desired p, i, n type semiconductor films respectively in the deposited film forming chambers 114, 115 and 116.
Also, in the above continuous deposited film forming device, although there are some advantages, the essential problems of the plasma CVD method still remain as described above.
Further, of the semiconductor devices, the present problems in a-Si semiconductor photovoltaic devices are lowering in cost and enhancement of efficiency. Particularly, the following points are obstacles against the problem of enhancement of efficiency.
(1) Since the carrier diffusion length in a-Si is too small, electrons, positive holes generated by light cannot be effectively collected in the p, n layer.
(2) Since the a-Si light absorption end energy is great, only a part of the natural sunlight spectrum can be absorbed to make the effective utilization percentage lower.
(3) Curved line factors due to series resistance are lowered as in large area device, etc.
Among these, for the purpose of solving the problem (1), the method of improving utilization efficiency of sunlight by dividing the sensitivity region of the sunlight spectrum into multi-layers is the most excellent, and researches of this method have been recently actively performed, partly because a multi-layer structure of a continuously connected structure (hereinafter called "tandem structure") can be easily formed in an amorphous semiconductor device.
Also, when forming a semiconductor device of a tandem structure as described above, the interface characteristics of the respective semiconductor film, particularly between the respective structural elements, for example, the interface characteristics between the respective nip elements in the case of a double layer structure device are important factors for energy converting efficiency, and there has been employed the method in which the respective semiconductor forming chambers are separated by vacuum in order to improve the interface characteristics and prevent mixing of the respective impurities during the reaction.
The bulk production type vacuum separation film forming device according to the plasma CVD method of the prior art for forming a double layer structure device of the substrate/nip/nip type according to the Roll-to-Roll method is disclosed in, for example, the above U.S. Pat. No. 4,389,970. The above mentioned problems are not also solved in the device described in said U.S. Patent.